In the process of manufacturing various semiconductor devices, a silicon oxide film [silicon oxide (SiO2) film], which is usable as, e.g., an insulating film, is formed by oxidizing silicon (mono-crystalline silicon or poly-crystalline silicon) on the surface of a semiconductor wafer. Conventionally, silicon oxide films are formed by, e.g., thermal oxidation processes. However, in the case of thermal oxidation processes, a thermal distortion may occur in a silicon substrate due to a high temperature that exceeds 1,000° C.
In light of this problem, there has been proposed a plasma oxidation processing method in which oxygen-containing plasma generated from a process gas containing oxygen gas is applied to a target object to perform an oxidation process of silicon (for example, International Publication No. WO 2004/008519).
According to a plasma oxidation process of the kind disclosed in International Publication No. WO 2004/008519 described above, it is thought that active species present in plasma, such as O radicals and O2+, act to cut Si—Si bonds and to generate Si—O bonds. The type and density of active species present in plasma differ according to plasma generating conditions, and have strong influences on results of plasma processes, such as the film formation rate (oxidation rate) and film quality of a silicon oxide film.
According to the conventional plasma oxidation process, since the absolute value of active species present in plasma can be hardly known, plasma process conditions to attain an optimum oxidation rate and required film quality are selected with reference to the state of a silicon oxide film formed by the plasma process.
However, this method entails a problem in that small fluctuations in the plasma process conditions interfere with expected plasma process results and vary the film thickness and/or film quality of a silicon oxide film due to a change in the oxidation rate, for example. Further, oxidation processes of mono-crystalline silicon are known such that the oxidation rate has plane direction dependence. According to the conventional plasma oxidation process, for example, the oxidation rate differs between the (110) plane and (100) plane of silicon, and the film thickness can be hardly uniform over portions having different plane directions. As described above, plasma oxidation processes involve difficulties in control different from those of thermal oxidation processes, and may cause semiconductor devices including a silicon oxide film as a component to suffer a decrease in yield and a variation in electrical performance, and thereby damaging the reliability.